A collimator is an optical device that converts light traveling within an optical fiber into a nearly collimated (pencil) beam of light propagating in free space. Arrays of collimators may have many applications in fiber-optic networks. Among them are digital, free-space, optical switches in which an input collimator is used to project a directed light beam from a fiber into free-space. This collimated beam may be aimed onto one or more moveable mirrors, which reflect the light to a chosen output collimator. The coupling efficiency between the input and output collimators may be strongly dependent on the angular accuracy with which the collimators are pointed. Thus, digital, free-space optical switches may require sets of collimators, i.e., collimator arrays, with very precise pointing accuracy. For state-of-the-art, digital, free-space optical switches, single collimator pointing accuracy of better than 150 micro-radians may be required. In order to provide flexibility in design and manufacturing, it is often desirable to adjust the collimator angle over a relatively large range (often one to a few degrees) with the above accuracy. An associated specification is the position accuracy, or “centration,” of the collimator. Centration refers to the positional accuracy of the center of the collimated beam relative to some reference point on the collimator. Even a precisely pointed (in angle) collimator pair may exhibit undesired optical coupling loss if their beam positions do not overlap precisely. It may be desirable to have a collimator with a very small centration error. For state-of-the-art, digital, free-space, optical switches, beam centration of better than 10 microns may be required.
A separate factor in coupling efficiency is the quality of the free-space beam emerging from the collimator. The collimator should not introduce significant aberrations because such defects will reduce coupling.
Another important attribute of a fiber optic collimator is to maintain the directionality of the light propagating in the fiber. An ideal collimator should have negligible back reflection. Typically, back reflection intensities should be 60 dB below that of the incident power.
In PCT International Publication Number WO 01/53860 A2, “PRECISION FIBER OPTIC COLLIMATOR,” Foster and Romanovsky describe the use of a plano-convex lens to collimate the beam emerging from an angle polished fiber. This reference is directed specifically toward so-called “analog” optical switches where the requirement for collimator alignment is simply to project the beam to the center of a specific moveable mirror. Because of the architecture of the switch (and the analog nature of its mirrors, which allow the mirror angle to be controlled effectively over a continuum of angles), any inherent beam position errors can be compensated by an adjustment of the beam angle, and vice-versa. The result is a collimator in which neither the angle, nor the position of the beam are accurately aligned, which is unsatisfactory for digital, free-space optical switches. This reference does not address a design in which both the angle and the position of the collimator are aligned.
A major disadvantage with the Foster and Romanovsky reference is that the angled surface of the fiber is not parallel to the plano surface of the lens. This leads to a complex alignment process, involving rotation of the fiber along its axis and/or rotation of the lens about its axis and/or translation of both the lens and fiber along their respective axes. In short, adjustment in as many as four degrees-of-freedom is required to align the beam in angle about two axes. While this approach allows for alignment of the pointing angle of the collimated beam relative to the outside of the fixture, it does not provide for a method to reduce the position error (centration error) below that of the plano-convex lens. In many real-world applications, even the centration error of lenses manufactured to state-of-the-art tolerances (tens of microns) is sufficiently large to lead to unacceptably high coupling losses.
A separate drawback of the Foster and Romanovsky reference is the finite gap between the fiber end and the plano surface of the lens. This gap is significantly larger than the typical Rayleigh range associated with the effective fiber aperture. Because of this, a spherical wavefront illuminates the plano surface of the lens. This leads to a substantial amount of spherical aberration, which degrades the quality of the beam.